Liquid cleaners are widely used in semiconductor manufacturing. One conventional wet process is the well-known RCA process, wherein a wafer is cleaned in hydrogen peroxide (H2O2) and ammonium hydroxide (NH4OH), followed with a solution of hydrogen peroxide and hydrogen chloride (HCl). Another conventional method, the Piranha clean, includes using a solution of sulfuric acid (H2SO4) and hydrogen peroxide (H2O2). Another conventional wet method uses an aqueous ozone solution. A basic requirement shared by all liquid cleaning methods is that the cleaning solvent must at least partially wet a surface.
The parameters known to control wettability include the liquid's surface tension, its contact angle with the surface, and the properties of the surface. Relevant surface properties include its surface energy, roughness, and its hydrophobic or hydrophilic character. Only when the contract angle is less than 90 degrees is a liquid generally considered to wet a surface. Problems associated with incomplete wetting include watermarks left by bubbles, incomplete cleaning, and distortion or collapse of narrow features from asymmetric meniscus forces.
In the past, the time required for the cleaning solution to wet a surface was not a major factor in process throughput. Recently, the introduction of ultrasonic stirring techniques, e.g. megasonics, further extended conventional cleaning methods for sub-micron technologies. However, as devices continue to shrink far below the 90 nm node, problems caused by wetting are predicted to become severe. In particular, cleaning high-aspect-ratio structures, such as DRAM trench capacitors and global interconnects are predicted to be especially problematic for wet chemical cleaning methods. A recent paper by M. T. Spuller et al., published in the Journal of The Electrochemical Society, 150(8), G467-G480 (2003) identified incomplete wetting of recessed nanoscale features as a potentially significant processing factor. This paper by Spuller et al. is hereby incorporated by reference in its entirety.
The analysis by Spuller et al., examined an entrapped gas bubble at the bottom of the recessed feature as illustrated in FIG. 1. By way of example, the recessed feature 10 may include an intermediate interconnect structure formed by etching through an interlevel dielectric 20 to an interconnect layer 30 in another dielectric layer 40. FIG. 1 demonstrates a problem caused by incomplete wetting, i.e. formation of a gas bubble 60. Unless the gas bubble is physically displaced, the only way for the cleaning solution 50 to fully wet the recessed feature is by dissolving the gas bubble 60. Spuller et al. estimated the wetting time td by deriving Equation 1 for 1D, diffusion-controlled conditions.
      Equation    ⁢                  ⁢    1    ⁢          :                  t      d        =                  (                  π          D                )            ⁢                        (                                                    HP                o                            ⁢              hw                                      8              ⁢              RT              ⁢                                                          ⁢              γcos              ⁢                                                          ⁢              θ                                )                2            
As shown by Equation 1, the time for the cleaning solution 50 to fully wet the recessed feature is td where D is the diffusivity of the gas in the liquid, h is the height of the feature, Po is the initial pressure of the gas in the feature (typically 1 atm), H is Henry's constant for the gas in the liquid, w is the width of the recessed feature, R is the ideal gas constant, T is the temperature, γ is the surface tension of the liquid, and θ is the contact angle of the liquid on the solid.
Contact angle θ is shown in FIG. 1. Note that as the contact angle approaches 90°, i.e. as the cleaning solution 50 becomes less wetting, Equation 1 predicts that the wetting time approaches infinity. While the prediction of an infinite wetting time may not be completely accurate, it is qualitatively correct to the extent it predicts that a given cleaning process may be economically unfeasible. For example, the wetting time for an aqueous cleaning solution on a hydrophobic surface having narrow recessed features may be so long as to render a previously used conventional process impractical.
In conclusion, incomplete wetting during semiconductor cleaning causes watermarks, incomplete cleaning, and distortion or collapse of narrow features. Problems such as these may only become worse as devices continue to shrink.